1. Field of the Invention
The present invention relates to circuitry for preventing doming phenomenon in a television receiver and a preventing method thereof.
2. Description of the Background Art
In a recent color television receiver, a cathode ray picture tube (CRT) is driven with a larger electron beam current than that in a conventional device in order to fulfill a demand for a reproduced picture of higher luminance. A phenomenon called "doming" occurs in driving with such a large beam current. This "doming" phenomenon will briefly be described.
FIG. 1A shows a schematic diagram of an entire cathode ray picture tube. Referring to FIG. 1A, the cathode ray picture tube includes an electron gun 500 for emitting electron beams of an intensity corresponding to a reproduced video signal, a deflection circuit 501 for generating a magnetic field for scanning the electron beams in the horizontal and vertical directions, a shadow mask 502 for selectively passing the electron beams, and phosphors 503 provided in an inner surface (a surface irradiated with the electron beams) of a faceplate (a phosphorescent surface) 504. The phosphors 503 each emitting a light of R (red), G (green) or B (blue) are provided in a predetermined sequence.
The shadow mask 502 comprises apertures for passing electron beams. Arrangement of the apertures of the shadow mask 502 is analogous to that of the phosphors 503 provided in the phosphorescent surface 504. Each of the apertures passes therethrough the respective electron beams corresponding to three kinds of colors, R, G and B to excite the associated phosphors of R, G and B for light emitting.
This shadow mask 502 absorbs approximately 80% of irradiating electron beams
Therefore, when a high luminance portion such as a white colored portion locally exists still for a long time (several tens seconds through several minutes), a portion of the shadow mask corresponding to that high luminance portion is irradiated with highly intense electron beams, so that the shadow mask 502 is partially, thermally distorted as shown in FIG. lB. The portions denoted with A in FIG. 1B schematically shows the form of the normal shadow mask, while the portions denoted with B schematically shows the form of the shadow mask when thermally distorted. As a result, mislanding of the electron beams occurs such that the electron beams not only irradiate their corresponding phosphors but also other phosphors through the apertures of the shadow mask 502, thereby causing miscoloring in a reproduced high luminance portion and thus a phenomenon that the high luminance portion such as a white portion is tinged with red or blue. This miscoloring in the high luminance portion such as a white portion due to the thermal distortion of the shadow mask is called a doming phenomenon.
The doming phenomenon tends to occur at the periphery, such as both sides of a display screen, but not in the center of the screen. This is because even if the thermal distortion of the shadow mask occurs at the center portion, the mislanding of the electron beams does not occur due to a small deflection angle of the electron beams therein; on the other hand, the periphery of the screen is largely affected by the thermal distortion of the shadow mask due to a large deflection angle of the electron beams thereto.
As described above, the doming phenomenon is caused by the high luminance portion such as the white portion locally standing still on the screen for a long time. Thus, the doming phenomenon occurs even if an average beam current (an average value of the electron beam current) through the cathode ray picture tube over the entire screen is at a predetermined value or less. Therefore, the doming phenomenon cannot be prevented by the conventional circuit for detecting an increase of the average beam current through the cathode ray picture tube over the screen and for immediately reducing luminance and/or contrast, i.e., an ABL (Automatic Brightness Limiter) circuit or an ACL (Automatic Contrast Limiter) circuit.
Such a configuration is disclosed in U.S. Pat. No. 4,096,518 that the electron beam current is limited by reducing luminance and contrast. In this prior art, an average electron beam current level is detected, and chrominance signal amplitude limitation and luminance signal amplitude limitation are simultaneously performed depending on the detected level, thereby limiting excessive beam currents.
A circuit configuration for limiting the electron beam current flowing through the cathode ray tube is disclosed in U.S. Pat. No. 4,587,554. In this prior art, a black level of a video signal is detected, thereby to change the specific rate of a reduction rate of a luminance level to that of contrast depending on the detected black level.
Another configuration is disclosed in U.S. Pat. No, 4,196,446 that the electron beam current is varied monotonously depending on the scanning distance of the electron beam with respect to the center of the screen.
In this prior art, the electron beam current is reduced monotonously depending on the distance from the center of the screen, thereby achieving uniformity in size of beam spots.